The present invention relates to a method of forming a pattern film by irradiating a focused ion beam to a predetermined region on a sample while blowing a film forming gas to a sample surface.
The conventional method of forming a pattern film due to focused ion beam irradiation is as follows. A film forming gas (for example, an organic compound gas such as pyrene) is blown locally to a predetermined region of a sample surface to be newly formed with a pattern film. Simultaneously therewith, a focused ion beam is irradiated while repeatedly scanning at the predetermined region. The film forming gas blown to the predetermined region is absorbed to the predetermined region of the sample and its vicinity of the sample surface. The organic compound absorbed at the predetermined region of the sample surface is dissolved by the irradiation of the focused ion beam, forming a carbon film. Because the focused ion beam is repeatedly scanned and irradiated, the carbon film is formed as a pattern film to a desired thickness. This pattern film is used to modify a mask used in semiconductor device manufacturing or to form a pattern (pattern modification) of a semiconductor device itself.
Also, if a metal organic compound gas is used as a film forming gas, a metal pattern is formed for interconnection, while, if a silicon compound gas is used, an insulation film is formed.
Incidentally, these organic compound gas, metal organic compound gas, silicon compound gas, etc. for forming a film due to focused ion beam irradiation are herein called a material gas.
If the repeated scanning of the focused ion beam is at a same number of times, the thickness of a film to be formed by this method varies depending on an area of a formed pattern film. If the area of the pattern film is great, the thickness of the pattern film becomes thick, while, if the area is small, the thickness becomes thin. This is because if the area of pattern film formation is broad, it takes a time for the focused ion beam to completely (one screen) scan over the pattern film forming region. As a result, there is an increase in the amount of a material gas absorbed to the sample surface until irradiated by the next focused ion beam scanning and the thickness of a pattern film to be formed increases. If the area of a pattern to be formed is small, the thickness of a pattern film decreases due to a reversal phenomenon. FIG. 4 shows part of such a situation. FIG. 4 is a graph showing a relationship between the area in which a pattern film is formed and the number of times of ion beam scans required for obtaining a thickness of a certain pattern film. A line 30 represents that the smaller the area the lower the film forming capability is.
Furthermore, if the area is small, etching effects due to focused ion beam irradiation becomes prevalent over formation of a film, resulting in no film formation or conversely cutting the substrate at the focused ion beam irradiating region.
As above, it is impossible to make constant the desired thickness of a film due to the area of a pattern film formed. In order to avoid this, first of all, where the area of a pattern film to be formed is less than a certain constant (for example, approximately 50 square xcexcm, xe2x80x9caxe2x80x9d in FIG. 4a), when a pattern film is formed by focused ion beam irradiation, a time interval is set of focused ion beam irradiation to a same point to cause the material gas to be sufficiently absorbed in the sample surface. That is, an irradiation suspension time is provided for each scanning of the focused ion beam by one screen over the pattern film forming region. The irradiation suspension time is set at same timing as such timing that a position when the film forming region is approximately 50 square xcexcm is again subjected to irradiation due to focused ion beam repeated scans.
However, when the pattern forming area is extremely small, the pattern film forming speed increases as shown by a line 31 in FIG. 4. This occurs when the width of the pattern forming region decreases almost as small as on the order of the diameter of the focused ion beam. That is, when the repeated scan with the focused ion beam is performed the same number of times to form a film, the thickness of the film increases as the area decreases. Although the cause of this is not known, part of the ion beam irradiated generally consumed by not film formation but in etching. Accordingly, film forming effects improves at a position of fine linewidth in terms of a same amount of material gas deposition.
Next, a relationship is previously determined under such a condition between the number of scans to obtain a given film thickness and the area of a pattern film to be formed, as in FIG. 4. The number of ion beam scans required for film formation is varied in accordance with the desired film area in compliance with the graph shown in FIG. 4.
The above conventional art is effective in the case where the film to be formed does not have both a portion with a broad pattern width and an extremely narrow portion. However, where the film to be formed has both a portion with a broad pattern width and an extremely narrow portion, when forming a film of a square large pattern form 21 having a pattern portion 21a in a projection form small in linewidth similarly on the order to the focused ion beam diameter as in FIG. 1, film forming efficiency increases at the pattern portion 21a in the projection form small in linewidth. The pattern portion 21a in the small projection form partially increases in film thickness, making it difficult to keep the film thickness constant throughout. There arises a problem where the film thickness is not desired to be unnecessarily high or the like.
Generally, the film formed has been partly etched by the focused ion beam. Thin films called halo are formed around the pattern film due to a substance of etching around the formed pattern film or scatter in part of the focused ion beam. As the thickness of the formed pattern film increases, the halo film thickness increases. Particularly in a technique of correcting mask patterns, there arises a problem of exposure because of serving as an ingredient to attenuate light. Assuming the film thickness of the pattern portion with a small linewidth projection form is made to a desired film thickness, the large area pattern portion is thin in thickness and difficult to sufficiently shield light, also raising a problem. Also, this halo problem poses a problem in changing interconnections because of reducing resistance values between neighboring interconnections.
In order to solve the above-stated problems, the region of ion beam scanning for film formation is decreased in every direction within a plane in a course of film formation.
According to the above method, the ion beam scanning region becomes limited to only a film central portion with a film thickness lower than a film periphery portion for an object to be formed with a film extremely narrow in linewidth. When the initial ion beam scanning region is extremely narrow, the ion beam scanning region itself vanishes, ending the film formation. Due to this, the film thickness can be suppressed from unnecessarily heightening at a fringe. Or, when simultaneously forming a plurality of films different of size, it is possible to suppress only a small film from abnormally heightening in film thickness.